SAI: 
SAI (Split APKs Installer) is an Android app that lets you install split APKs (such as ones distributed as Android App Bundle). It has both rooted and rootless installation methods.
SAI is kinda dead cause I have no motivation for continuing its development. I will probably maybe still occasionally fix bugs and add small features/improvements, focusing on the installer functionality, but idk. If you're looking for an actual backup solution, you can try OAndBackupX or Swift Backup.
Please read Contributing guide
SAI adds some meta information to .apks files it exports, you can find the description of the format it uses in the Meta format description
SAI is licensed under GPLv3